In wireless communication systems, a transmitted signal is distorted due to dynamic properties of a radio channel through which it is transmitted. In order to compensate for the dynamic properties of the radio channel, different methods are available for combating interference. An ideal compensation would completely cancel the effects of the radio channel and the resulting equalized channel would be completely frequency flat. However, such a scheme would in most cases lead to unwanted noise amplification limiting the performance. Equalization schemes must therefore provide a trade-off between noise amplification and making the equalized channel frequency-flat.
For the transmitted data to be recovered at the receiver it is important that the interference is suppressed. Besides the power consumption aspect of the user equipment, there is also a desire to restrict the size and costs of the user equipment in order for it to be attractive. The desire to reduce size, cost and power consumption is valid also for receivers in the base station. The space for and costs of processing circuitry should therefore be kept at a minimum. The complexity of the methods used for combating the interference competes with a desire to cancel the interference to as large extent as possible.
Normally, the channel is estimated, where the estimation is based on so-called pilot sequences. A pilot sequence is a sequence that is known to both ends in a communication system. A consequence is that parts of the air interface resources (spectrum, time, codes) are occupied with pilots, and can then not be used for transmitting data. However, transmitting pilot sequences enables the system to optimize the bandwidth used for transmitting user data.
Each pilot sequence is only transmitted during certain time and frequency periods, and a consequence of this is that as time progresses, the estimate becomes less good due to a variation of the channel. Differently stated, it means that the error of the channel estimate increase as a function of time. One implication of an error increase is that the user data through-put decrease. Also, pre-coders suffer in terms of less reliable weights. That is, there is a larger uncertainty regarding where to place the antenna beam energy.
In case an underlying channel model is used to interpolate intermediate channel outcomes, it is crucial to change the model based on environment and user behaviour. This results in computationally complex algorithms in which various parameters such as Doppler shift, Doppler spread and delay spread are needed components. The quality of the estimation is, naturally, depending on how well the model can describe the underlying channel behaviour.
In view of the above, it is desirable to provide an enhanced estimation of a channel by using pilot sequences, which is not as vulnerable to passing time as for presently employed channel estimation.